Donald inherited his father's titles in 1387 and spent the early
years of his tenure suppressing a revolt by his younger brother John Mór
who, with the support of the Macleans, was trying to expand on his small
inheritance of lands. By 1395 Donald had driven John Mór into exile in
Ireland and beaten off the challenge of the Macleans.

Donald was then able to turn his attention to the more serious
problem facing him. The Stewarts were very effective producers of offspring,
both legitimate and illegitimate, and this meant that the family consumed more
and more territory as one after another of Robert II's children were granted titles
and associated lands. As a major landowner, and despite being
Robert II's grandson, Donald would have
seen this as a growing threat.

Matters came to a head over the Earldom of Ross, which controlled a
vast swathe of northern Scotland extending from the
Isle of Skye to
Inverness, and therefore
extended right along the northern flank of Donald's territories. Partly to
secure his northern flank, Donald had married Mariota Leslie, sister of
Alexander Leslie, the Earl of Ross. But when Alexander died in 1402, Mariota's
claim to the Earldom was overlooked in favour Alexander Leslie's sickly young
daughter, Euphemia.

By now control of Scotland was nominally in the hands of Donald's
cousin, Robert III, who succeeded to
the throne in 1390. He was a deeply ineffective King, and the real power lay in
the hands of his ruthless and ambitious younger brother,
Robert, Duke of Albany. As soon as
the young Euphemia inherited the Earldom of Ross,
Robert, Duke of Albany, who was the
girl's grandfather, took control of her and assumed the title of "Lord of the
Ward of Ross": clearly a step en route to taking over the title in its
entirety.

Robert, Duke of Albany
assumed personal control of Scotland on the death of
Robert III in 1406 as the latter's
son, 12 year-old James I, was a prisoner
in England, where he would remain until 1424. Donald responded by exchanging
messages with both James I in captivity
and with Henry IV of England, presumably seeking their support in an attack on
Robert, Duke of Albany.

In Summer 1411 Donald, Lord of the Isles, gathered an army of
10,000 men including MacIntoshes, Macleans, Macleods, Camerons and Chattans as
well as MacDonalds, and took control of the Earldom of Ross by force. He then
marched towards Aberdeen where
forces were being raised against him by Alexander Stewart, Earl of Mar, a
nephew by adoption of Robert, Duke of
Albany. The two armies met at the
Battle of Harlaw on 24
July 1411, with Donald's 10,000 highlanders being confronted by around 2,000
better equipped men under the Earl of Mar, most of whom were cavalry. At the
end of the day's fighting, Donald withdrew, having lost up to 1,000 of his men:
compared with the much larger proportionate losses of 600 suffered by the Earl
of Mar.

It remains open to debate who, if anyone, "won" the
Battle of Harlaw. The
person who came out best was not actually present on the day. After Donald
withdrew his forces, Robert, Duke of
Albany, was able to proclaim the outcome as a triumph for the forces of
civilisation over the barbarious highland hordes. He then led an army to retake
Ross unopposed. In 1415, he "persuaded" Euphemia to relinquish the Earldom of
Ross, which he awarded to his son, John Stewart, the 2nd Earl of Buchan.
Euphemia spent the rest of her days in a nunnery.

In 1412, Robert, Duke of
Albany followed up his success in Ross with preparations to invade the
Lord of the Isles'
heartland. He held off doing so after Donald agreed to the terms of the Treaty
of Lochgilphead. No
details of this remain, but it seems likely that Donald was allowed to keep his
existing lands, in return for giving up any claim on the Earldom of Ross.

Robert, Duke of Albany,
died in 1420, and Donald, Lord of the Isles, died in 1423. Donald was succeeded
his son, Alexander of Islay, who became the
third Lord of the
Isles. Robert's death cleared the main obstacle to the return of
James I from captivity in England, and
James assumed the throne in 1424, his
first act being to wipe out the surviving Albany Stewarts, one effect of which
was to revert the Earldom of Ross to the Crown.